Mycoplasma hominis infection of Trichomonas vaginalis is not associated with metronidazole-resistant trichomoniasis in clinical isolates from the United States.

Trichomonas vaginalis is a protozoan parasite that is the cause of the most common non-viral sexually transmitted disease, trichomoniasis. Metronidazole and tinidazole are the only drugs approved for treatment of T. vaginalis infections in the USA. However, drug resistance exists and some patients are allergic to these medications. Furthermore, the exact mechanism of metronidazole resistance remains undefined and current testing methods require several weeks before results are available. Identification of the mechanism of drug resistance may lead to the development of molecular tools to detect drug resistance, and quicker results for clinical treatment. In a recent study, Chinese T. vaginalis isolates that were polymerase chain reaction (PCR) positive for Mycoplasma hominis DNA demonstrated greater in vitro resistance to metronidazole than isolates with no evidence of M. hominis infection. To evaluate this finding in isolates from a distinct epidemiologic setting, we tested 55 T. vaginalis isolates collected from patients in the USA through the Centers for Disease Control and Prevention metronidazole susceptibility testing service. One half of the isolates demonstrated resistance to metronidazole by an in vitro sensitivity assay. Of the metronidazole-resistant T. vaginalis isolates, 18% were PCR positive for M. hominis, as were 22% of the metronidazole-susceptible T. vaginalis isolates (p = 0.746). We also observed no change in metronidazole sensitivity of two infected T. vaginalis isolates after they were cleared of their M. hominis infection by culturing the isolates in antibiotics. Thus, M. hominis infection of USA T. vaginalis isolates did not correlate with in vitro resistance to metronidazole.